Shading correction is a technology for controlling the effect on an image of a dropping in the amount of peripheral light that is a characteristic of lenses. In general, the shorter the focal distance of the lens (the wider the angle) or the smaller the f-stop, the greater the peripheral light dropping. As a result, it is undesirable to apply the same correction conditions to all image sensing conditions (that is, zoom positions and f-stops)
For example, Japanese Laid-Open Patent Application Publication Nos. 2000-41179 and 8-294133 disclose a mechanism that pre-stores correction conditions for every possible image sensing condition that can arise and obtains a correction value based on the image sensing conditions obtained from the lens. This conventional structure is described using FIG. 4.
FIG. 4 is a block diagram showing a structural example of those portions that relate to shading correction in a conventional image input apparatus.
In the image input apparatus described above, light passing through a zoom lens 401, a stop 402 and a focus lens 403 is converted by an image sensing device 405 into an electrical signal at each pixel, and, after being read out in sequence by an image sensing device drive circuit 407, is converted into a digital signal by an A/D circuit 406. At the same time, shading correction data corresponding to the f-stop, zoom position and lens type is pre-registered in a ROM1 (413), a ROM2 (414) and a ROM3 (415), and a controller 412 supplies address signals corresponding to the pixel positions to these ROM.
The controller 412 acquires the zoom lens 401 image sensing conditions and supplies them to a ROM data switching unit 416. Depending on the zoom lens 401 image sensing conditions received from the controller 412, the ROM data switching unit 416 outputs to a multiplier 408 the output value of that ROM has the appropriate data from among ROM 1-3 (413-415).
The multiplier 408 performs shading correction for each pixel by multiplying the digital signal from the A/D circuit 406 by the shading correction value that is the output of the ROM data switching unit 416. The corrected pixel signal is then subjected to predetermined image processing by a signal processing circuit 409, such as color interpolation, white balance, encoding and the like, and the processed image is then displayed on a display device 410 and/or recorded on a recording medium such as a memory card or the like by a recording medium recording unit 411.
Thus, as described above, by obtaining and registering in advance correction values for every possible combination of lens type, zoom position and f-stop, in theory suitable shading correction is possible. In practice, however, with the structure described above it is difficult to be able to perform proper shading correction under all image sensing conditions.
Under the image sensing conditions described above, there are not very many combinations of lens type and f-stop, and if the apparatus is a fixed lens type the number of f-stops alone will suffice. However, the number of values that can be derived from the zoom position alone is very great. Particularly with recent increased demand for high picture quality there is also an increasing emphasis on optical zoom capabilities that provide zooming without deterioration in picture quality, and therefore high-magnification zoom lenses have come to be used. Even a compact digital still camera commonly mounts at least a 3× optical zoom lens, and even 6×-10× optical zoom lenses are not unusual. In addition, video cameras mount at least an 8× optical zoom lens, and usually a 10-12× optical zoom lens.
In devices that use such high-power optical zoom lenses as these, it is very inconvenient to prepare correction values for all possible zoom positions. In addition, shading correction is performed at each pixel of the image sensing device, and therefore, as the number of pixels packed onto a single image sensing device continues to increase to the point where image sensing devices with pixel densities of 4-6 million pixels are now common, it can be easily understood that the amount of shading correction data involved for a single combination also increases dramatically.
In order to eliminate the inconvenience of preparing correction values in advance and to economize on the memory capacity required to store those correction values, it is possible to perform correction by selecting a limited number of zoom positions for which correction values are prepared and using the correction value that corresponds to the prepared zoom position that is closest to the actual zoom position. However, recently, and particularly with electronic still cameras, in order to make such devices smaller and thinner, there is a trend toward increasingly strong demand for smaller and thinner optical systems including optical zoom lenses as well. As a result, shading (a drop in the amount of light at the periphery of the lens) increases with distance from the optical axis of the lens, and furthermore, the shading characteristics due to zoom position also tend to fluctuate greatly. Under these circumstances, using a correction value corresponding to a different zoom position reduces correction accuracy.
For example, in the case of an optical zoom lens having shading characteristics like those shown in FIG. 5, correction values are prepared for discrete positions intermediate between the wide-angle end (Wide) and the telephoto end (Tele) (for example, the four positions Wide/M1/M2/Tele). For zoom positions for which no correction values are provided, the correction value that is closest to that zoom position is selected from among the group Wide, M1, M2, Tele, as well as the correction value of the zoom position in the wide-angle direction.
In this case, assume, for example, that the optical zoom lens magnification (zoom position) is changed continuously from Wide to M1. In this instance, at intermediate zoom positions a, b the Wide correction value is applied. As a result, however, proper correction is not performed for intermediate zoom positions.
FIG. 6 shows shading-corrected luminance levels at these intermediate positions (normalized to the luminance at the center of the optical axis). As shown in the diagram, when the zoom position is at Wide, M1, proper shading correction suited to each of these positions is performed, so that there is no difference in luminance level between the center of the optical axis and periphery. As the zoom lens moves from Wide in the telephoto direction and reaches position a, shading correction corresponding to Wide is performed. In this case, as indicated by “a” in FIG. 6, the luminance level increases from the center of the optical axis (point 0) toward the periphery, in a state of over-correction.
As the zoom position moves further in the telephoto direction and reaches position b, over-correction due to application of the correction value corresponding to Wide increases further. Then, when the zoom position moves from b to M1, shading correction appropriate for M1 is performed and a flat luminance level at M1 like that at the center of the optical axis is obtained.
Thus, as described above, using shading correction values corresponding to a position in the wide-angle direction for an intermediate zoom position leads to over-correction, and conversely, using shading correction values corresponding to a position in the telephoto direction for an intermediate zoom position leads to under-correction. As a result, for example, when carrying out moving-image image sensing while zooming continuously from Wide to M1, a switch between an image that has been properly corrected and the post-corrected image becomes evident as an abrupt change in luminance in the sensed moving image. Such image switching includes, for example, a switch from an error-corrected image at an intermediate position to a properly corrected image at M1.
Thus, as described above, with the conventional structure, achieving proper correction at a given zoom position entails either inordinate inconvenience and memory capacity consumption or allowing error correction.